The present invention relates to a transformer for lighting a discharge tube such as a neon tube or an argon tube, in particular, to a discharge tube lighting transformer with a protective circuit against non-grounding of a ground terminal thereof when the transformer is installed.
FIG. 1 shows a conventional transformer of the kind described above, which is disclosed in FIG. 11 of U.S. Pat. No. 6,504,691 (issued Jan. 7, 2003). A leakage transformer 11 includes a primary winding 12, one end of which is connected through a switch 13 to a hot line terminal 14 and the other end of which is connected to a non-active line terminal 15. An a.c. power source such as a commercial power supply is connected between the hot line terminal 14 and the non-active line terminal 15, and the ground terminal of the a.c. power source 10 is connected to the non-active line terminal 15. The leakage transformer (hereafter simply referred to as a transformer) 11 also includes a secondary winding 16 having a mid-point 17 which is connected to a ground terminal 18. A sign lamp (discharge tube) such as a neon tube, an argon tube or the like is connected between the opposite ends 16A and 16B of the secondary winding 16. Where the transformer 11 has a metal casing 11a, the ground terminal 18 is provided on the casing 11a. When the transformer 11 is installed on a neon tower or the like, the ground terminal 18 is grounded through a lead wire 18a. 
When there is a voltage in excess of a given value between the non-active line terminal 15 and the ground terminal 18, it is detected by a non-grounding detection circuit 21. Specifically, one end of a capacitor 22 is connected to the ground terminal 18 while its other end is connected to the cathode of a diode 23 and to the anode of a diode 24. The cathode of the diode 24 is connected to one end of a capacitor 26 through a resistor element 25 while the anode of the diode 23 and the other end of the capacitor 26 are connected to the non-active line terminal 15, and a junction between the diode 24 and the resistor element 25 is connected to the non-active line terminal 15 through a Zener diode 27. A junction between the resistor element 25 and the capacitor 26 is connected to the base of a transistor 28 acting as a switching element, and the emitter of the transistor 28 is connected to the non-active line terminal 15 through a Zenor diode 29. It is to be understood that the resistor element 25 and the capacitor 26 can be omitted.
An input portion of the non-grounding detection circuit 21 is a rectifier circuit including the capacitor 22 as an input. In this example, a rectifier circuit 31 is formed by the capacitors 22 and 26 and the diodes 23 and 24. When a rectified output voltage of the rectifier circuit 31 exceeds a given value, the transistor 28 is rendered conductive.
When the non-grounding detection circuit 21 detects a voltage which is equal to or greater than a given value, interrupter means interrupts the supply of the a.c. power to the primary winding 12. At this end, the collector of the transistor 28 is connected to the hotline terminal 14 through a light emitting element 33L of a photocoupler 33 and through a path including resistor elements 34 and 35 and a rectifying diode 36. A series circuit including a light receiving element 33P of the photocoupler 33 and a relay 37 is connected between the hot line terminal 14 and the non-active line terminal 15, and the switch 13 is formed by a changeover switch which is formed by contacts of the relay 37. When the relay 37 is operated, a movable contact MC of the switch 13 is thrown to a normally open contact NO, whereby the hot line terminal 14 is connected to the relay 37 through the normally open contact NO of the switch 13, thus forming a self-holding circuit of the relay 37.
The leakage transformer 11 is constructed so that voltages induced across the windings 16a and 16b disposed on the opposite sides of the mid-point 17 of the secondary winding 16 are unbalanced with respect to the mid-point 17. For example, the transformer may be constructed such that two magnetic circuits which pass magnetic fluxes resulting from a flow of a current through the both secondary windings 16a and 16b have responses which are different from each other. As shown in FIG. 2, the primary winding 12 is disposed centrally on one side of a magnetic core 71 which is in the form of a rectangular frame while the secondary windings 16a and 16b are disposed on the magnetic core 71 at locations which are disposed on the opposite sides of the primary winding 12, with leakage cores 72 and 73 being provided at locations between the primary winding 12 on one hand and secondary windings 16a and 16b, respectively, on the other hand for shunting the magnetic path of the magnetic core 71. In the example shown, the leakage cores 72 and 73 have widths t1 and t2, respectively, which are different from each other to provide different magnetic flux leakage responses, thus making magnetic circuits 74 and 75 which pass magnetic fluxes resulting from a flow of a current through the secondary windings 16a and 16b to be mutually unbalanced. t1 may be by 10 to 30% less than the width t of the magnetic core 71 while t2 may be by 10 to 30% greater than t.
A secondary wiring may be passed through a flexible metal tube commonly referred to as a metal conduit in order to prevent a fire casuality. In this instance, a metal conduit assumes a ground potential, and accordingly, there is a current flow though slightly through a capacitance of the conduit. This is illustrated in FIG. 3 where it will be noted that the output terminals 16A and 16B are grounded through capacitances CS1 and CS2, respectively, and accordingly, a very week current flows to the ground through these capacitances CS1 and CS2. It then follows that the output terminals 16A and 16B are connected to the ground (reference potential) through high impedance elements which are based on the capacitances CS1 and CS2.
In this arrangement, if the ground terminal 18 is actually grounded, a potential difference between the non-active line terminal 15 and the ground terminal 18 is equal to zero, and accordingly, no voltage is applied to the non-grounding detection circuit 21. Accordingly, the transistor 28 remains non-conductive, and the relay 37 cannot be operated and the movable contact MC of the switch 13 is thrown to the normally closed contact NC, and thus the a.c. power from the terminals 14 and 15 are supplied to the primary winding 12.
However, if the a.c. power is applied across the terminals 14 and 15 when the ground terminal 18 is not grounded, the ground potential which the metal conduit assumes becomes a reference, and potentials at the output terminals 16A and 16B are less than a maximum potential Vmax by voltage drops V1 and V2, respectively, across the capacitances CS1 and CS2, and a potential V3 which is equivalent to such reduction occurs at the mid-point 17. An ordinate 92 in FIG. 3 represents a position from the mid-point 17 to the terminal 16A and 16B, and an abscissa 93 represents a potential as referenced to the ground potential, and a curve 91 depicts a potential on the secondary winding 16. The ordinate and the abscissa cross at point 94, which corresponds to the mid-point 17, and which assumes a potential of zero. However, due to differences in the magnetic responses of the magnetic circuits 74 and 75, a voltage is developed at the mid-point 17 of the secondary winding 16 even though the magnitude of the voltage is small. As a consequence, when the ground terminal 18 is not grounded, a voltage is developed between the ground terminal 18 and the non-active line terminal 15, and is rectified by the rectifier circuit 31. When the rectified output voltages becomes greater than a sum of the Zener voltage of the Zener diode 29 which may be assumed to be 12V and the base-emitter voltage of the transistor 28, which may be assumed to be 0.6V, for example, the rectified output from the rectifier circuit 31 renders the transistor 28 conductive, whereby there occurs a current flow through the light emitting element 33L to emit light, which causes the light receiving element 33P to operate the relay 37, throwing the movable contact MC of the switch 13 to the normally open contact NO, thus interrupting the supply of the source power to the primary winding 12 and maintaining such condition by the self-holding circuit of the relay 37. Accordingly, if one intends to operate the discharge tube lighting system when he has forgotten grounding the ground terminal 18, the supply of the source power is automatically interrupted, and the lighting system cannot be operated.
Rather than using different widths for the leakage cores 72 and 73, different lengths G1 and G2 may be used for magnetic air gaps in order to produce different flux leakage responses of the leakage cores 72 and 73. Alternatively, both the widths t1 and t2 and the lengths G1 and G2 may be different from each other. Alternatively, the secondary windings 16a and 16b may have slightly different lengths, thus shifting the mid-point 17 slightly. What is required is that the magnetic circuits 74 and 75 have different magnetic responses. However, when an unbalance between the magnetic circuits 74 and 75 is too high, it may have an adverse influence upon the lighting response of the sign lamps. In sum, it is essential that the magnetic responses of the magnetic circuits 74 and 75 which are formed by the secondary windings 16a and 16b, respectively, be by an amount on the order of ±10 to 30% greater or smaller than those of the conventional magnetic circuits 74 and 75 which have an equal magnetic response.
When a conventional non-grounding protective circuit is used, in the event an operating personnel has forgotten to ground the ground terminal 18, no a.c. power is supplied to the primary winding of the transformer, thus assuring the safety. However, when installing the discharge tube lighting transformer, if an operating personnel inadvertently connects the hot line terminal 14 to the ground side of the a.c. power source 10 and connects the non-active line terminal 15 to the non-grounded side of the a.c. power source 10 or when the connection is made in a wrong polarity, the non-grounding protective circuit becomes operative, ceasing to supply the a.c. power to the primary winding 12. Specifically, when the connection is made in the wrong polarity, if the ground terminal 18 is grounded, the non-active line terminal 15 has a potential which changes in a sinusoidal form to the positive and the negative polarity with respect to the ground potential of the a.c. power source 10. Accordingly, when the non-active line terminal 15 assumes a negative potential with respect to the ground terminal 18 which is grounded, the a.c. power which then prevails is rectified by the rectifier circuit 31 to provide a rectified output, which renders the transistor 28 conductive, causing the light emitting element 33L to emit light and thus operating the relay 37, whereby the a.c. power can no longer be supplied to the primary winding 12.
A mistake may occur in the wiring due to a troublesome wiring work, and the polarity of the a.c. power source 10 may be unclear sometimes, and therefore there has been a need that an operating personnel examine the polarity of the a.c. power source 10 in order to avoid a wiring in the wrong or inverse polarity. An operating personnel is demanded each time the wiring work is made to see if the connection has been made in the right polarity, but it is possible that the operating personnel forgets the need of such inspection, and there may be no output from the secondary winding 16 as a result of the connection in the inverse polarity, but an inadvertent decision may be rendered that this has occurred as a result of a failure of the transformer 11 itself. In addition, an inspection which takes place in a factory is made by connecting discharge tubes 19 in series across the output terminals 16A and 16B without using a metal conduit and without grounding the mid-point of a series connection or the mid-point of the load. Again, if the ground terminal 18 is not grounded, a reference potential point on the secondary side cannot be fixed, for example, allowing the voltage occurring at the mid-point 17 to vary in an unstable manner. Where a secondary a.c. output voltage is equal to 15 kV, it is possible that the voltage at the mid-point 17 may be on the order of 100V, causing the non-grounding detection circuit 21 to operate. Accordingly, it must be assured during the inspection that ground terminal 18 be grounded. Also during the inspection, if the connection between the a.c. power source 10 and the hotline terminal 14 and the non-active line terminal 15 is made in a wrong polarity, the non-grounding detection circuit 21 may operate. Accordingly, it must be assured that the ground terminal 18 is grounded and that the connection with the a.c. power source 10 is made in a right porality also during the inspection, requiring an increased length of time for such inspection. In addition, there is a likelihood that the operating personnel may forget to assure this, to foul the inspection.
In the prior art practice, the non-grounding detection circuit 21 is provided for the reason to be described below. When a discharge tube lighting transformer is installed on a neon tower, for example, if a wiring for the discharge tubes 19 happens to be in contact with the neon tower while the discharge tubes on the neon tower are illuminated, a high tension a.c. power may flow to the neon tower, which represents the ground, presenting a risk of causing a fire. In order to detect such a so-called ground fault, the mid-point 17 of the secondary winding has been connected to the ground terminal 18 through a ground fault detection circuit 38, as indicated in broken lines in FIG. 1. A specific example of the ground fault detection circuit 38 will be described later with reference to FIG. 4. A corresponding circuit is shown in FIG. 9 of the U.S. Pat. No. 6,504,691 as designated by reference numeral 41.
This ground fault detection circuit 38 cannot detect a ground fault if the ground terminal 18 is not grounded. The purpose of the non-grounding detection circuit 21 is to prevent this from occurring. However, the conventional non-grounding detection circuit has difficulties which were described above.
It is an object of the present invention to provide a discharge tube lighting transformer with a no-grounding protective circuit which does not operate for a wiring in the inverse polarity, but which operates upon occurrence of a ground fault if the ground terminal is not grounded.